Conventionally, a NOx storage-reduction catalyst has been known as a catalyst to reduce and purify a nitrogen compound (NOx) in exhaust gases discharged from an internal combustion engine. The NOx storage-reduction catalyst stores NOx contained in exhaust gas when the exhaust gas is in a lean atmosphere and reduces and purities the NOx that is stored therein with hydrocarbons contained in the exhaust gas when the exhaust gas is in a rich atmosphere to make the NOx harmless for discharge into the atmosphere. Since the NOx storage capacity of the catalyst is limited, a so-called NOx purge needs to be executed periodically in which exhaust gas is made rich by performing a post injection or an exhaust pipe injection to restore the NOx storage capacity of the catalyst (for example, refer to Patent Literature 1).